| 摘要 |
A gain-controlled amplifier amplifies an input signal at a gain which varies exponentially in accordance with a control voltage applied thereto and selectively outputs one of non-inverted and inverted output signals substantially identical in characteristic with each other. The gain-controlled amplifier comprises first and second transistor pairs, a common emittor driving circuit and a subtracting circuit. The first transistor pair comprises first and second transistors, and the second transistor pair comprises third and fourth transistors. The emittors of each of the transistor pairs are joined together, and bases of the first and third transistors and bases of second and fourth transistors are joined together, respectively. The two sets of joined bases serve as input terminals of the control voltage. The common emitter driving circuit complementalily drives each of the joined emitters in accordance with the input signal of the amplifier. Connected to the collectors of the first and third transistors and the collectors of the second and fourth transistors are a current-mirror circuit and resistors, respectively, as loads. A current is supplied to the collector of the first transistor in accordance with the input signal. The subtracting circuit subtracts a signal appearing at a collector of one of the second and fourth transistors from a signal appearing at a collector of the other of the second and fourth transistors, and outputs the subtraction result as the output signal of this amplifier.
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